1. Field of the Invention
This invention relates to the equivalent circuit of an environmental sensor, the resistance of which changes depending on the state of an object to be detected in the ambient atmosphere (that is, in an environment), namely, depending on, for example, a gas or an odor, like a gas sensor for detecting a gas generated when a fire occurs, for instance, a CO-gas and an odor sensor (namely, an odorant sensor) for detecting a smoky smell or odor when a fire brakes out. Further, this invention relates to a method for measuring an odor pressure, a method for normalizing an odor pressure, an odor detection system and a fire detection system.
2. Description of the Related Art
There has been known a conventional environmental sensor for detecting the state of an odor pressure (i.e., the concentration or strength of an odor) in the environment thereof, which comprises a sensitive film, the resistance of which changes according to the variation in odor pressure (corresponding to an odor intensity, an odor quantity, a gas pressure or a gas concentration) of an object to be detected, and a heater for heating this sensitive film. FIG. 8 is a sectional view of a conventional environmental sensor, for example, a thin-film odor sensor. In this figure, reference numeral 1 designates the odor sensor which comprises an insulating substrate (for instance, an alumina substrate) 2, a sensor electrode 3 mounted on a surface, for example, the top surface 2a of this alumina substrate 2, a sensitive film 4 consisting of a metal oxide semiconductor (for example, SnO.sub.2, ZnO, TiO.sub.2 or the like) deposited or vapored on both of the top surface 2a of the alumna substrate 2 and the surface of the sensor electrode 3 and having resistance which changes according to the odor pressure, a heater electrode 5 mounted on another surface, for instance, the bottom surface 2b of the alumina substrate 2, and a thin film serving as a heater (for instance, a platinum thin film) 6 which is deposited on both of the bottom surface 2b of the alumina substrate 2 and the surface of the heater electrode 5. This platinum thin film 6 heats the sensitive film 4 through the alumina substrate 2. This promotes the change in resistance of the sensitive film 4 depending on the odor pressure, to thereby increase the sensitivity of the film 4.
The conventional odor sensor 1 is constituted as described above. However, there are two types of the sensitive films 4, namely, n-type and p-type films. In the case of the n-type of the sensitive film 4, when an odor pressure is exerted thereon, it accepts an electron to change its resistance. In contrast, in the case of the p-type of the sensitive film 4, when an odor pressure is exerted thereon, an electron is extracted from the sensitive film 4, with the result that the resistance thereof changes. Further, the odor sensor 1 is one for detecting an odor by utilizing the resistance change which depends on the odor pressure. It has been well known from, for example, a paper described by P. K. Clifford (see Proceeding of the International Meeting of Chemical Sensors, Fukuoka, Sept, 1983, A113) that the power law given by the following equation (1) can be generally applied to the odor-pressure-sensor-resistance characteristics. EQU Z=AP.sup.-a ( 1)
where Z represents a total resistance of the sensitive film 4 of the odor sensor 1; P an odor pressure; and A a proportional constant. Incidentally, an exponent "a" varies with odors and meets the following inequality: 0&lt;a&lt;1.
As stated above, in the case of the conventional odor sensor, the exponent "a" varies with odors and is smaller than 1. Thus, the conventional odor sensor has drawbacks in that it is very difficult to treat a compound odor and in that it is impossible to measure a partial odor pressure of a specified odor in the compound odor whose composition is unknown.
The present invention is accomplished to eliminate such drawbacks of the conventional environmental sensor.
Accordingly, an object of the present invention is to provide a specified equivalent circuit of an environmental sensor which can eliminate the drawbacks of the conventional environmental sensor.
Further, another object of the present invention is to provide a method for measuring an odor pressure by obtaining and utilizing a resistance r.sub.n which changes on the basis of the odor pressure in such a manner that the exponent "a" becomes equal to 1.
Moreover, a further object of the present invention is to provide a method for normalizing the odor pressure.
Furthermore, still another object of the present invention is to provide an odor detection system which utilizes such methods.
Additionally, yet another object of the present invention is to provide a fire detection system which utilizes such methods.